A wide variety of coatings have been used to coat the surfaces of food and beverage containers. For example, metal cans are sometimes coated using coil coating or sheet coating operations, that is, a coil or sheet of steel or aluminum, is coated with a suitable composition and cured. The coated substrate is then formed into the can body or can end. Alternatively, the coating composition may be applied, for example, by spraying and dipping, to the formed can and then cured. Coatings for food and beverage containers should preferably be capable of high speed application to the substrate and provide the necessary properties when cured to perform in a demanding end use environment. For example, the coating should be safe for food contact and have excellent adhesion to the substrate.
Many of the coating compositions for food and beverage containers are based on epoxy resins that are the polyglycidyl ethers of bisphenol A. Bisphenol A in container coatings either as bisphenol A itself (BPA) or derivatives thereof, such as diglycidyl ethers of bisphenol A (BADGE), epoxy novolak resins and polyols prepared with bisphenol A and bisphenol F are problematic. Although the balance of scientific evidence available to date indicates that small trace amounts of BPA or BADGE that might be released from existing coatings does not pose health risks to humans, these compounds are nevertheless perceived by some as being harmful to human health. Consequently, there is a strong desire to eliminate these compounds from coatings for food and beverage containers. Accordingly, what are desired are container coating compositions for food and beverage containers that do not contain extractable quantities of BPA, BADGE or other derivatives of BPA and yet have commercially acceptable properties.
Hydroxyl and carboxylic acid functional polyesters curable with aminoplast or phenolplast curing agents provide suitable container coating compositions. However, such compositions can exhibit poor heat-aging properties. Specifically, they suffer from poor humidity resistance when the cured coating is exposed to high humidity conditions at elevated temperatures. When coated substrates such as the interior of carbonated beverage cans or can ends are exposed to such conditions and tested with a carbonated aqueous solution of citric acid, phosphoric acid and salt, known as L-85 liquor, the polyester/aminoplasts perform poorly. Consequently, the beverage manufacturers, although desirous of replacing the epoxy resins based on bisphenol A with polyesters, will not accept this poor performance.
The present invention discovered the poor performance of the coating due to heat aging was catalyzed by acid functionality in the polyester and if acid capping agents were reacted with the polyester resin, the degradation of the coating performance could be retarded, thereby maintaining performance of the coating. This modification results in a cured coating with improved cured coating performance when tested in accordance with the L-85 testing protocol described in paragraphs “G” and “H” appearing on pages 15 and 16 of this application.